There are PV diodes that short in the middle of the board. There is a power FET that can short or fail that seems to control board power up. They can lose the ability to see DCS, the processor just fails. The one 35V Cap fails and either pops or expands and pushes off can. They have power and create 5V and PV voltage, but just dead with no current draw. There are other faults like shorted fets for lights, audio amp blows, motor fet shorts.
If one shorts but is left powered up, it shorts until the component fails open. At which point it looks dead with no current draw.
There really is no way to know for sure unless a failure analysis was done. The design engineer might have insight, but they have moved on to different stuff.
This board was a 2000 to about 2004 run. Granted a lot of trains built in that time frame, but there are solutions if they fail.
Trying to figure out how to prevent a failure is like trying to determine how you will prevent a stone chip while driving your new car to work every day on the highway. These boards are about 18 years old. 
G
